Four-Winged Dinosaurs and the Dawn of Flight

XING XU
The ancestors of birds may have taken to the air on four wings and a prayer. Paleontologists have recovered from deposits in Liaoning, China, dinosaur fossils that exhibit evidence of flight feathers on their hindlimbs as well as their forelimbs. The specimens are said to represent a long-sought intermediate stage in the evolution of birds from flightless theropod dinosaurs, and could breathe new life into the theory that protobirds glided between trees before developing powered, flapping flight.

Evolutionary biologists have long debated whether birds began winging it by gliding among the trees or by racing along on the ground. The latter scenario has gained favor in recent years. But the new finds, described in a report published today in the journal Nature, "provide negative evidence for the ground-up hypothesis" and instead support the arboreal gliding scenario, assert study author Xing Xu of the Chinese Academy of Sciences and his colleagues.

The remains, which date to roughly 130 million years ago, include a new species of dinosaur dubbed Microraptor gui (above). A member of the dromeosaurid family of dinosaurs--the group to which birds are thought to be most closely related--the diminutive Microraptor gui apparently bears a striking resemblance to a creature whose existence was predicted nearly 90 years ago. American naturalist William Beebe proposed in 1915 that the earliest bird was a four-winged glider, or tetrapteryx. Microraptor gui, notes University of Kansas paleontologist Richard O. Prum in a commentary accompanying the report, "looks as if it could have glided straight out the pages of Beebe's notebooks."

Xu and his collaborators base their conclusions on the long aerodynamic feathers that cover the fore- and hindlimbs of Microraptor according to the same pattern seen on modern bird wings. Long feathers around the ankles would have made traveling on the ground difficult. But the forelimb and hindlimb feathers "would make a perfect aerofoil together," the authors write, likening it to the membrane employed by bats and gliding animals.

Still, Prum cautions that more questions about Microraptor remain, such as how the creature could have rotated its legs in order to use its hindwings. He further points out that the presence of four wings may have been unique to dromeosaurs like Microraptor, rather than being a trait present in the last common ancestor of birds and dromeosaurs.

"The Origins of Birds and Their Flight," by Kevin Padian and Luis Chiappe (Scientific American, February 1998), is available for purchase at the Scientific American Archive.